Ask a local veterinarian to visit your class and talk about pet health care or some interesting cases. Maybe he/she could bring some xrays to hang in a sunny window. Maybe some cool pictures or samples of internal parasites. 3-5 graders love to be grossed out. Most kids don't realize that vet hospitals usually keep a blood donor cat around the place.
Have the students think of questions they want to ask before the veterinarian visits.